This proposal outlines a positional cloning strategy for the gene responsible for Hereditary Hemorrhagic Telangiectasia (HHT) or Osler- Weber-Rendu disease. This congenital disorder is characterized by multisystemic vascular dysplasia and recurrent hemorrhage leading to serious morbidity and 10% mortality. However, the nature of the primary biochemical defect and the location of the gene remain unknown. The varied features of this disorder suggest the gene product may play a critical role in blood vessel structure and development. The proposal is divided into four successive specific aims. 1. Genetic linkage analysis will be performed using highly polymorphic microsatellite repeats and large multigenerational families segregating HHT. These pedigrees, which have already been collected and sampled, represent the critical resource for this proposal. Preliminary analysis has already excluded over 70 Mb of the genome. 2. Chromosome microdissection will be used to narrow the candidate region by providing probes for refined linkage analysis and seed probes for physical mapping. A yeast artificial chromosome contig will be established in the region to provide complete coverage of the region in cloned DNA. 3. Candidate genes within the region will be identified by cDNA cloning using whole YACs, CpG islands, evolutionarily conserved sequences, exon trapping, and sequences identifying a transcript on Northern blots of endothelial and vascular smooth muscle RNA. Expression studies and partial sequencing will determine their potential for involvement in HHT. In addition, a Southern blot survey for alteration in HHT patients will be used to identify the most promising candidate genes. 4. The HHT gene will be identified by searching for mutations within each candidate gene in patients. Mismatch chemical-cleavage will be used to identify differences in normal and patient DNA, with subsequent sequencing of the patient's gene to confirm the mutation. The identification of the HHT gene will provide new insight into the pathology of this disorder as well as the biology of the vascular system. This study will provide the molecular framework for future studies on the role of the HHT gene product in blood vessel structure and development.